Understanding aperture priority with a configurable AF confirm chip

[ghostman]

I just got an old Takumar 50mm f/1.4 lens, an m42 adapter and a Canon AF confirm chip (http://store.tagotech.com/product_info.php?cPath=22_37&products_id=139). The AF confirm chip is configurable, so I can change the reported aperture, focal length and even change the front/back focus while the chip is mounted onto my XTi.

I'm using this in aperture priority mode (Av) and I'm getting confused about something. The documentation for the AF chip says that the reported aperture is mainly for "information purpose only". That makes sense, since the aperture on the lens is manual, the aperture the chip reports doesn't change my actual aperture.

So, I set my lens wide open @f/1.4. I configure the chip to report back f/1.4. I'm in Av mode, so the shutter speed returns 1/200. Now, without changing the actual aperture on the lens, I configure the chip to report back f/2.8. In Av mode, the same subject now returns a shutter speed of 1/80. The exposure on the two shots are different.

I don't get it. Why does my shutter speed change, when the true aperture on the lens did not? I thought Av tells me the shutter speed based on the available light that it senses.

[PhotosGuy]

I thought Av tells me the shutter speed based on the available light that it senses. It reports available light that it senses, which is affected by the value of the subject it's reading, the Exposure Compensation (EC) you dialed in, & metering modes, and so I don't use it except for special situations. This shows how the subject can affect the exposure & why manual keeps me worry free:
Post #47 (http://photography-on-the.net/forum/showpost.php?p=5191658&postcount=47)
Click the "Thread: (thread title)" link at the top-right if you'd like more info on the subject.

[ghostman]

It reports available light that it senses, which is affected by the value of the subject it's reading, the Exposure Compensation (EC) you dialed in, & metering modes, and so I don't use it except for special situations. This shows how the subject can affect the exposure & why manual keeps me worry free:
Post #47 (http://photography-on-the.net/forum/showpost.php?p=5191658&postcount=47)
Click the "Thread: (thread title)" link at the top-right if you'd like more info on the subject.

That's definitely an interesting thread, but in that situation the photographed scene changes (however slightly).

I had my camera mounted onto my tripod the whole time with the scene unchanged (I can layer one shot over the other). It was indoors, so the light remained the same. I was using partial metering, no EC changes.

I'd assume that the returned shutter speed would remain the same if the actual aperture doesn't change. I guess I'll try it with manual mode next.

[egordon99]

The camera assumes that it will be able to actually stop the lens down to f/2.8 (had it beeen an EF lens) at the time of exposure. It can't, and you didn't actually move the aperture ring, so you ended up with a shot that was ~2 stops overexposed.

With EF lenses, the lens is wide open during focusing and metering, so when it reads the light TTL, it's coming in as "bright" as f/1.4, but if you select f/2.8, it then "dials down" the shutter speed by two stops because it will stop the lens down to f/2.8 at the time of exposure, thus giving you the proper exposure.

Does that make sense?

I just got an old Takumar 50mm f/1.4 lens, an m42 adapter and a Canon AF confirm chip

So, I set my lens wide open @f/1.4. I configure the chip to report back f/1.4. I'm in Av mode, so the shutter speed returns 1/200. Now, without changing the actual aperture on the lens, I configure the chip to report back f/2.8. In Av mode, the same subject now returns a shutter speed of 1/80. The exposure on the two shots are different.

I don't get it. Why does my shutter speed change, when the true aperture on the lens did not? I thought Av tells me the shutter speed based on the available light that it senses.

[ghostman]

The camera assumes that it will be able to actually stop the lens down to f/2.8 (had it beeen an EF lens) at the time of exposure. It can't, and you didn't actually move the aperture ring, so you ended up with a shot that was ~2 stops overexposed.

With EF lenses, the lens is wide open during focusing and metering, so when it reads the light TTL, it's coming in as "bright" as f/1.4, but if you select f/2.8, it then "dials down" the shutter speed by two stops because it will stop the lens down to f/2.8 at the time of exposure, thus giving you the proper exposure.

Does that make sense?

Thanks. That makes sense. But that assumes the camera knows the TTL light it is reading is coming in at f/1.4 and that f/2.8 is a stopped-down aperture, no? What if my lens was wide open at f/2.8, but getting as much light as if it were f/1.4 (meaning, I had more light in the room)? Then there would be no need to adjust the shutter speed. Perhaps my logic is off somewhere.

In any case, it seems it's not true that the configured aperture is for "information purpose only". A manual lens should always be configured to its largest aperture. That way, a wide open lens would get proper exposure in Av mode. And when the lens is stopped down, the camera can think it's still at f/1.4 but in a darker scene and adjust the shutter appropriately.

[tonylong]

As far as I can understand from what you're saying, the "reporting" chip is reporting what an EF lens reports about its maximun aperture. So, when you change the "reporting" you are telling the camera the aperture is now at f/2.8 so the camera takes the light reading with the available (f/1.4) light and adjusts the shutter speed to work with f/2.8.

It sounds like if you want to use AE with the lens, you will have to both change the reporting aperture and the physical aperture prior to metering. Otherwise you'll get a high amount of light from the f/1.4 physical aperture.

Something like that!

[ghostman]

As far as I can understand from what you're saying, the "reporting" chip is reporting what an EF lens reports about its maximun aperture. So, when you change the "reporting" you are telling the camera the aperture is now at f/2.8 so the camera takes the light reading with the available (f/1.4) light and adjusts the shutter speed to work with f/2.8.


Perhaps I'm just dense (very possible), but when it takes the light reading with the available (f/1.4) light, how does it know it's not taking the reading at f/2.8 already so that no adjustment is necessary?

For example, assume I have two different 50mm prime lenses, each with different max apertures:
Scenario 1: 50mm f/1.4@f/1.4 with available light. This yields shutter speed = 1/200 for a properly exposed photo.
Scenario 2: 50mm f/2.8@f/2.8 with additional light introduced. This also yields a shutter speed = 1/200 for a properly exposed photo.

Both scenarios will yield 1/200 shutter speed because of the extra lights added in Scenario 2. Now, I attached both lenses to the same m42 adapter with the same af confirm chip configuration with their respective scenarios. The af confirm chip's "reporting" aperture is set to f/2.8. The camera knows nothing of the actual lenses since all the information it gets come from the AF chip, so the camera believes the same lens is attached in both scenarios, right?

Scenario 1 (my current scenario): With the AF chip, the shutter speed is adjusted to 1/80 and overexposes the photo. The assumption is that the camera knows that, in order to reach f/2.8 as the chip requires, the lens must be stopped down and the shutter speed must be slowed to still provide the proper exposure. Because the lens never gets stopped down, the photo is overexposed with the decreased shutter speed.

Scenario 2 (fictional scenario): With the AF chip, would the shutter speed also be slowed to 1/80?
If so, why? If the camera knew that the lens must be stopped down to reach f/2.8 in Scenario 1, wouldn't it also know that the lens does not have to be stopped down here? And if the camera does not know better, why did it adjust the shutter speed in Scenario 1?
If not, why not? Is the camera able to tell a different lens is mounted, even though there is no communication between the body and the lens?


Sorry for being so thick. I know I'm wrong somewhere and I'm certain I'll slap my forehead afterward.

[tonylong]

I haven't messed with this type of lens, so I'm just taking a stab, but here's my take:

Your camera doesn't know whether you are shooting with an f/1.4 or f/2.8 lens. When you set the chip to report f/2.8, that is the aperture that the camera assumes. So, when the meter reads the light, it suggests a shutter speed that corresponds to f/2.8 to give a "good" exposure. So, if you are using an f/1.4 lens, that suggested lens speed will give you an overexposure. It doesn't matter how much light there is, just that the camera "believes" you are working with an f/2.8 aperture, meters the scene, and gives a shutter speed that will work with f/2.8.

It's the same as if you were shooting with an EOS lens using manual exposure and were using an f/1.4 lens, and set your aperture to f/2.9 -- the camera would suggest a shutter speed that would work with f/2.8 (even though it is reading the light through the f/1.4 aperture). Then, if you opened up the aperture to f/1.4 but didn't adjust the shutter speed, you would get the same overexposure. You are shooting with the shutter speed suggested for f/2.8, not f/1.4.

Does that make sense?

To shoot with this setup, you would want to set the chip for your intended aperture, close the lens to that aperture, and let the camera meter accordingly, or set the chip to the widest aperture, do the reading, then do the math for stopping down the aperture before stopping the lens down and resetting your shutter speed.